Factsheet
Roadside Research Project
Department of Horticulture
College of Agricultural Sciences
Roadside Vegetation Management
http://rvm.cas.psu.edu
5
Managing Japanese Knotweed and Giant Knotweed on Roadsides
Untangling the Truth About
Knotweed
Japanese knotweed (Polygonum cuspidatum,
[known to some botanists as Fallopia japonica]), also
known as 'bamboo', is a tall-growing, hollow-stemmed,
perennial herbaceous plant capable of forming dense
monocultures on a wide range of site conditions, from
sterile road-cuts to stream banks (Figure 1). Knotweed
is not a true bamboo (a woody, evergreen grass), but is
a relative of plants such as buckwheat, smartweed, and
the Noxious Weed mile-a-minute vine. Japanese
knotweed and its close kin giant knotweed (Polygonum
sachalinense) (Figure 2) are native to East Asia, and
were introduced to the U.S. for use as ornamentals
during the late 1800’s. However, due to their imposing
height (6 to 10 ft), dense growth habit, aggressive
spread, and seeming indifference to control methods,
the knotweeds have become a pervasive weed in a
number of settings.
Knotweed stems emerge in late-March to mid-April,
depending on soil temperatures, and begin a burst of
rapid growth. Roadside knotweed can already be tall
enough to cause sight distance problems before the
end of April, and the plants are almost full height by the
end of May. Flowering usually occurs in July, and the
seeds mature in August and September.
Figure 1. Showy flowers of Japanese knotweed. The
knotweeds were introduced to the U.S. in the late 1800's as
ornamentals due their prominent late-season flower display
and striking height. Due to its great size, knotweed reduces
sight distance and encroaches on the travel lanes, particularly
on secondary roads.
Knotweed reduces sight distance and damages
pavement along roads (Figures 1 and 3), overruns lowto-no maintenance areas, and crowds out existing plant
communities.
As frightening as the aboveground growth of
knotweed is, it is the rhizome system that is the root of
the problem. A rhizome is an underground stem that
gives rise to roots, aerial stems, and more rhizomes.
Knotweed rhizomes spread vigorously, expanding the
size of the knotweed stand. Rhizomes are also very
durable. A very small piece of rhizome that is moved to
another site will give rise to a new plant. Roadside
maintenance such as ditch cleaning is one the primary
ways that knotweed is spread.
Control Methods
The key to controlling knotweed is controlling the
rhizome system of the plant. Knotweed is one of those
plants best thought of as being like an iceberg - what
Figure 2. Japanese knotweed (left) and giant knotweed (right)
occur throughout Pennsylvania. The leaves of Japanese
knotweed are usually 4 to 6 inches long, while the leaves of
giant knotweed can reach 12 inches in length and are
distinctly heart-shaped. Though often confused with each
other, there is little chance of confusing these imposing plants
with any other species. The knotweeds have similar
adaptations and growth habits, and can be regarded as
equally problematic.
By Art Gover, Jon Johnson, and Larry Kuhns, 2005. This work was sponsored by the Pennsylvania Department of Transportation. The contents
of this work reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents do not
necessarily reflect the official views or policies of either the Commonwealth of Pennsylvania or The Pennsylvania State University, at the time of
publication. This work does not constitute a standard, specification, or regulation.
you can see is only the tip of the problem. Control
measures that fail to address the regenerative capacity of
the rhizomes will not control this plant.
The best approach is to use as many methods as
possible so that each operation adds to the effect of
previous operations. This is a simplified way of describing
Integrated Vegetation Management (IVM). The following
sections describe the different approaches that can be
used - ideally used together in a pre-planned program,
rather than repeated instances of 'fire-fighting'.
Control methods are categorized as mechanical,
cultural, biological, or chemical. Your program should rely
on as many methods as possible in a pre-planned
approach.
Well-established knotweed is very difficult to control,
and successful control will require a multiple-step 'control
phase', and an ongoing 'maintenance phase' in the
following seasons to make sure the knotweed does not reestablish. For example, the control phase could consist of
a cutting followed by an herbicide application, or an earlysummer herbicide application followed by a fall application.
Figure 3. Knotweed shoots emerging through asphalt.
Knotweed not only limits sight distance, it physically
damages shoulders and road edges.
Mechanical Control
Cutting alone is not an effective tool for managing
knotweed. However, cutting can be an integral part of
managing this plant, particularly when combined with a
follow-up herbicide application.
Knotweed is herbaceous (non-woody), so though it
may be imposing, the stems cut quite easily. Where
terrain permits, knotweed can be mowed. Where mowing
is not feasible, motorized trimmers with metal blades, or
hand tools such as a machete or stout sickle will suffice. A
properly timed cutting will eliminate the tall canopy and
make follow-up operations much easier. Our experience in
Pennsylvania suggests targeting June 1 as a cutting date.
The regrowth from this cutting date is vigorous, but limited
in height. A June 1 cutting of 6 to 10 ft tall knotweed
results in regrowth that is only 2 to 4 ft tall.
Cutting too early in the season can result in regrowth
that reaches full height. This effect is often seen when
knotweed growth is killed by a spring frost. Cutting later
than June reduces your operational window to chemically
treat knotweed, and waiting too late in the season can
result in almost no regrowth. We have conducted lateAugust mowings and had almost no regrowth to treat.
Cultural Control
Cultural methods can be described as indirect, or
preventive weed control. One example of a cultural control
method for knotweed is preventing the spread of rhizome
pieces in soil and on excavation equipment.
Encouraging or establishing alternative groundcover
provides competition to knotweed, and will enhance the
effects of other treatments. However, Japanese knotweed
appears to inhibit the growth of other plants. In trials
where Japanese knotweed growing in a monoculture has
been controlled, only a few weed species seem to colonize
Figure 4. A high-volume foliar application is the best
method to treat knotweed that has not been cut. This
application should be made after July 1 and before a killing
frost.
the former knotweed area the next growing season.
Giant knotweed appears to cause less inhibition, and we
have been able to revegetate these areas with coolseason grasses. Revegetating treated knotweed areas
with grasses allows you to use herbicides that will injure
the knotweed regrowth but not harm the grasses.
Chemical Control
Herbicides can be applied to the plant foliage or to
the surrounding soil. Stem injection is advocated for
some areas, but this is extremely laborious. Stem
injection is only viable where spraying is prohibited, and
the amount of knotweed is limited.
Foliar Applications
The most common herbicide application is to spray
the foliage. This is a productive method, and more
selective than soil applications to control knotweed.
Three factors that will determine the success of a foliar
application are application timing, spray equipment, and
the herbicides selected.
Managing Japanese Knotweed and Giant Knotweed on Roadsides - 2
Application Timing
Herbicide selection
Our current understanding of knotweed growth is
that the first vigorous flush of growth is fueled by stored
carbohydrates in the rhizomes. During this growth spurt,
the net movement of carbohydrates is to the canopy away from the rhizomes. The herbicides used for a foliar
application move through the plant with the
carbohydrates. To control the rhizomes, the application
needs to be made later in the season, when net
movement of carbohydrates is back to the rhizomes for
growth and storage. By 'later in the season', we mean
the window from July 1 to the first killing frost.
Glyphosate is the herbicide of choice for controlling
knotweed. It is effective, has no soil activity, can be
used near water, it is readily available, and inexpensive
(Table 1).
If the knotweed is going to be cut around June 1 as
described above, a follow-up herbicide application
should be delayed for at least six weeks. This allows the
knotweed to resprout and develop its new (though
smaller) canopy, and begin sending carbohydrates back
to the rhizomes.
Spray equipment
The choice of equipment depends on accessibility,
and size and density of the knotweed stand. In narrower
rights-of-way (ROW), a fixed-pattern applied by a spray
truck is certainly the easiest and quickest way to treat
knotweed. On wider ROW where the knotweed is
beyond the reach of a spray truck, uncut knotweed is
best treated with a high-volume application using a
handgun on a length of hose (Figure 4). If the uncut
knotweed stand is large enough that you will be required
to walk through it - as opposed to being able to treat it
from the perimeter - you should count on spray volumes
of 200 to 300 gallons per acre to achieve 'spray-to-wet'
coverage.
When knotweed has been cut and the shorter
regrowth is being treated, a backpack sprayer may be
the tool of choice. The shorter canopy can be effectively
treated with a low-volume spray (5 to 40 gallons per
acre), and the backpack provides the applicator more
mobility than dragging a hose and handgun. An
additional benefit is that non-target vegetation is much
easier to see and avoid during treatment.
Arsenal (imazapyr) and Vanquish (dicamba) have
demonstrated significant activity on knotweed, but
have appreciable soil activity and should not be used
near desirable trees. Garlon 3A (triclopyr) has not
been effective against established knotweed - it injures
the topgrowth but does not appear to injure the
rhizomes. However, triclopyr is safe to grasses and
has aquatic labeling in ROW settings. It may be useful
as a follow-up treatment where desirable grasses,
sedges, or rushes are present.
Soil Applications
Spike 20 P (tebuthiuron) is a pelleted herbicide
that is applied to the soil. This product is non-selective
and soil active. The effects of a tebuthiuron application
develop slowly. It may be well into the summer before
you are convinced that a spring application was
effective. Tebuthiuron remains active in the soil for
more than a year and is readily taken up by the roots of
all plants, including nearby trees. The treated site
should begin to revegetate during the summer of the
season after a spring application.
This product would have a fit only in a wide ROW
where the treatment will not impact desirable trees,
result in erosion, or pose a threat to water.
Implementing a Knotweed
Management Plan
Due to the difficulty of initially treating knotweed,
and the persistence of its rhizomes, effective control
requires a programmed approach that includes initial
control operations and ongoing maintenance.
Two Phase Program
Control of any persistent weed requires an
effective control operation and ongoing maintenance to
Figure 5. Our current view of the knotweed management calendar, based on its seasonal growth and response to control
measures. In the roadside setting, the greatest concern is the period described as 'management needed, but operations are less
effective', when knotweed is tall enough to impact sight distance, but mowing treatments will tend to result in tall regrowth and
herbicide applications will only burn knotweed back, at best.
Managing Japanese Knotweed and Giant Knotweed on Roadsides - 3
prevent the weed from re-establishing. The chances of
totally eliminating a weed from an area are slim because
there is usually more of it just on the other side of the
ROW fence. To be effective, your program must target
knotweed when it is most susceptible to control
measures (Figure 5).
Control Phase
The effectiveness of the control phase will determine
how much effort is required in the maintenance phase.
Better control means less follow-up work. Therefore,
effort and resources should be front-loaded into a
multiple-operation control effort.
A common approach may be to treat uncut knotweed
one time, late in the summer, with a high volume foliar
application. This is an effective treatment, but thorough
coverage of an extensive knotweed stand is difficult.
There is a good chance there will be misses and some
resprouts. You should schedule a follow-up treatment
before the end of the season, and apply the initial high
volume treatment before August 1 to allow time for
resprouting to occur. If you are going to apply only once,
delay the treatment to late-August/early-September
window, after flowering, to maximize herbicide
movement to the rhizomes.
• 'Chemical mow'/Treat: Target knotweed in lateApril/May during the broadleaf weed control
program with mixture such as Garlon 3A or
Vanquish plus Escort XP (metsulfuron), or Garlon
3A plus Vanquish (Table 1). These mixtures will
stunt and burn back the knotweed foliage. This
will improve sight distance. Retreat with
glyphosate in the late summer.
Maintenance Phase
A follow-up application the first season after the
control treatments is critical. The timing of maintenance
herbicide applications follows the same calendar as the
control applications - after July 1.
After this initial maintenance cycle, the visits can be
scheduled on a two-year cycle if the treatments are
successful. The maintenance applications can be
applied using a backpack sprayer because targets will
be limited. This allows the applicator to be selective in
targeting, minimizing the damage to non-target plants.
If the non-target vegetation is mostly grasses,
sedges, rushes, or other non-broadleaf species, you can
enhance selectivity by using broadleaf herbicides such
as Vanquish or Garlon 3A (Table 1).
SUMMARY
Here are some other control phase treatment
scenarios:
• Late Cut/Treat: Cut around June 1, followed by a
glyphosate application between August 1 and the
killing frost. The advantage of this late cut is that
the knotweed regrowth is short. The drawback is
that the knotweed was tall enough to negatively
impact the roadside for at least a month.
Established knotweed stands are extremely
tenacious. A well-executed control program will
eliminate the existing stand along with the roadway
hazards it presents for the short-term. Maintenance
visits to the site to follow-up on resprouts are necessary
to prevent re-establishment, and to allow an alternate
groundcover to develop.
• Multiple Cut/Treat: Cut twice in the spring to
prevent the knotweed from hindering sightdistance followed by a glyphosate application
between August 1 and a killing frost.
The routine maintenance program will only suppress
knotweed, at best. Successful management of knotweed
on the ROW requires special programming.
Table 1. Herbicides on the current (2005) Pennsylvania State Herbicide Contract that are effective for treating knotweed, listed by
application technique.
Treatment
Foliar
Soil application
Products
Comments
Glyphomate 41, GlyPro Plus
(glyphosate)
Best choice. Apply at 4 qt/ac. Due to aquatic labeling, Glyphomate 41
is preferred when applying near water.
Arsenal (imazapyr)
Significant soil activity. Do not use near desirable trees.
Vanquish (dicamba)
Soil activity. Safe to grasses, but nearby trees can be injured.
Garlon 3A (triclopyr)
Not effective for initial control - only seems to control topgrowth. Due to
safety to grasses and aquatic labeling may be useful as a follow-up in
sensitive settings.
Escort XP (metsulfuron)
Not effective by itself. However, Escort XP is a standard ingredient in
treatments targeting broadleaf weeds and brush that could be used to
'chemically mow' knotweed early in the season.
Spike 20 P
Suitable only where knotweed is not near desirable trees or adjacent to
water. Effects are slow to occur, and long lasting. Treated areas may
be devoid of any vegetation for almost two growing seasons.
Managing Japanese Knotweed and Giant Knotweed on Roadsides - 4